Read-Only type disks such as compact disks, Write-Once type disks whereon data can be written only once, and Rewritable disks such as magneto-optical disks, are widely known as optical disks.
To write or read data on or from an optical disk such as mentioned above, a light beam, for example a laser beam, needs to be projected and converged accurately on recording tracks formed on the optical disk. In the conventional art, the 3-beam method or the push-pull method is predominantly adopted for executing the tracking control of the light beam. With the 3-beam method, a main beam that is irradiated on the center of a track and a pair of sub beams that are irradiated slightly shifted in opposite directions off the center of the track on both sides of the track, are produced and employed. Meanwhile with the push-pull method, only one light beam is used and divided in two by means of an optical system, and the quantities of light of the two light beams produced are compared. The 3-beam method permits a finer tracking control than the push-pull method, when the disk is warped or when the depth of the guiding grooves forming the track is irregular.
A conventional optical head device whose optical system comprises a diffraction grating element provided with diffraction gratings created by the use of interference fringes of two divergent lights, and that employs the 3-beam method for the tracking control will be described below with reference to FIG. 38.
In a conventional optical head device, a light emitted from a semi-conductor laser 101 is divided into a main beam (i.e. zeroth order diffracted light) and a pair of sub-beams (i.e +1 order and -1 order diffracted lights) by a first diffraction grating element 102, and is directed to a second diffraction grating element 103. The sub-beams are located in a plane approximately orthogonal to the surface of the paper, form a predetermined angle with respect to the main beam and stretch away in opposite directions. The three beams are further diffracted in the second diffraction grating element 103 and only the zeroth order diffracted light beams produced as each of the three beams are diffracted, pass through a collimating lens 104 and are converged on a recording medium 106 by an objective lens 105.
As illustrated in FIG. 39(a), at this time if the recording medium 106 is for example a compact disk, a main beam M (accurately the zeroth order diffracted light produced by the main beam) is converged on a recording track 108 formed on the recording medium 106 so as to read data recorded in the form of a pit 109 on the recording track 108. Data is obtained and read from the luminous intensity of the reflected light of the main beam M. A focusing error signal is also derived from the main beam M, as it will be described later.
Meanwhile, two sub beams (accurately the zeroth order diffracted lights produced by the sub beams) S.sub.1 and S.sub.2 are converged in positions relatively far apart from each other in opposite directions with respect to the main beam M in the track direction Y of the recording medium 106, and slightly shifted in opposite directions with respect to a direction X. The direction X represents a radial direction of the recording medium 106 as well as the diffracting direction of the second diffraction grating element 103 to be described later (the direction perpendicular with the track direction Y and hereinafter referred to as the diffracting direction X). A tracking error signal is derived from the luminous intensities of the reflected lights of the two sub beams S.sub.1 and S.sub.2.
The main beam M reflected from the recording medium 106 and the reflected lights of the sub beams S.sub.1 and S.sub.2 pass through the objective lens 105 and the collimating lens 104. Only the first order diffracted lights produced as the reflected lights are diffracted by the second diffraction grating element 103 in the diffracting direction X, are directed to a photodetector 107. Hereinafter, the first order diffracted lights produced in the diffraction grating element 103 by the main beam M will be referred to as main first order diffracted lights, and the first order diffracted lights produced by the pair of sub beams S.sub.1 and S.sub.2 as the pair of sub first order diffracted lights.
The configuration of the diffraction gratings formed in the second diffraction grating element 103 as seen from the recording medium 106 side, is illustrated in FIG. 40(a) and the configuration of photo-detecting parts 107a to 107f formed in the photodetector 107, as seen from the recording medium 106 side, is illustrated in FIG. 40(b). As illustrated in FIG. 40(a), the second diffraction grating element 103 is composed of two diffraction gratings 103a and 103b divided by a join line 103c that is parallel with the diffracting direction X and intersects an optical axis L (shown in FIG. 38). Grating lines 103d are formed in the diffraction grating 103a, and grating lines 103e are formed in the diffraction grating 103b such that the grating lines 103d and grating lines 103e have a different grating pitch and are formed in a direction substantially perpendicular to the join line 103c.
As to the photo-detector 107, it is divided into six photo-detecting parts 107a to 107f such that the longitudinal direction of each of the photo-detecting parts 107a to 107f is parallel with the diffracting direction X. When the main beam M projected on the recording medium 106 is converged correctly without any focusing error, the main first order diffracted light produced in the diffraction grating 103a is converged on a parting line 107g and forms a spot P.sub.1 '. The main first order diffracted light produced in the diffraction grating 103b is converged on a parting line 107h and forms a spot P.sub.2 '. The sub first order diffracted lights are converged on the photo-detecting parts 107e and 107f respectively.
Supposing that Sa to Sf represent the output signals released by the photo-detecting parts 107a to 107f, a focusing error signal FES may be determined by performing the operation FES=(Sa+Sd)-(Sb+Sc). A tracking error signal RES is determined by performing the operation RES=Se-Sf, and a recorded data signal RS by performing the operation RS=Sa+Sb+Sc+Sd.
The diffraction gratings 103a and 103b are usually composed of grooves and have a rectangular profile as illustrated in FIG. 41(a). However, diffraction gratings having a serrated profile that permit a high optical utilization efficiency, are also being studied.
In the above arrangement, the two spots P.sub.1 ' and P.sub.2 ' are formed on the photodetector 107 fairly apart from each other along the diffracting direction X. Therefore, the photo-detecting parts 107a and 107c are aligned and disposed longitudinally along the diffracting direction X. Consequently, the photo-detecting parts 107b and 107d are juxtaposed and disposed longitudinally along the diffracting direction X. This causes the entire photo-detector 107 to extend considerably lengthwise along the diffracting direction X and thereby to occupy a large space, and the fabrication cost to increase.
When each the diffraction gratings 103a and 103b have a serrated profile, in order to form the two spots P.sub.1 ' and P.sub.2 ' fairly apart from each other along the diffracting direction X, the angle of diffraction at the diffraction grating 103a and the angle of diffraction at the diffraction grating 103b need to differ from each other considerably. In other words, the respective grating pitches of the diffraction gratings 103a and 103b need to differ from each other considerably. This not only causes the fabrication process of the diffraction gratings 103a and 103b to be complex but also, creates a difference between the optical utilization efficiencies as the diffraction gratings 103a and 103b have a different profile, thereby causing an accurate focusing error detection to be infeasible. The profile of each of the diffraction gratings 103a and 103b may be formed so as to reduce the difference between the optical utilization efficiencies. However, such a profile does not correspond to the optimum profile desired for each of the diffraction grating 103a and 103b thereby causing the optical utilization efficiency to lower and the quality of reproduced signals to drop.
Furthermore, when using an optical head device where the 3-beam method is adopted in order to write/read data on a Write-Once type disk, the following difficulties arise. Suppose that the recording medium 106 shown in FIG. 39 is an optical disk of the Write-Once type where recording tracks 108 are previously formed in the shape of guiding grooves or the like. When reading data, the pair of sub beams S.sub.1 and S.sub.2 are irradiated on pits 109 formed on a recording track 108 in a substantially identical fashion, as illustrated in FIG. 39(a). The variation in the reflectivity occurring due to the pits 109 is thus substantially identical for both sub beams S.sub.1 and S.sub.2 thereby permitting the tracking error detection to be executed smoothly. However when writing data as illustrated in FIG. 39(b), the sub beam S.sub.2 that precedes the main beam M is irradiated on an unrecorded section while the sub beam S.sub.1 that follows the main beam M is irradiated on a previously formed pit 109. As a result, the sub beams S.sub.1 and S.sub.2 have a different reflectivity even if the main beam M is positioned on the center of the recording track 108 causing the accuracy of the tracking error detection to lower.